The amino acid glutamate (L-glutamic acid) is recognized as the major excitatory neurotransmitter in the CNS. The excitatory amino acid receptors which respond to glutamate are of great physiological importance and play a key role in a variety of physiological processes such as long-term potentiation (learning and memory), the development of synaptic plasticity, motor control, respiratory and cardiovascular regulation, and sensory perception.
Excitatory amino acid receptors are classified into two general types and both are activated by glutamate and its analogs. Receptors activated by glutamate that are directly coupled to the opening of cation channels in the cell membrane of the neurons are termed ionotropic glutamate receptors (iGluRs). This type of receptor has been subdivided into three subtypes, which are defined by the depolarizing actions of the selective agonists N-Methyl-D-aspartate (NMDA), α-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and Kainic acid (KA).
The second general type of glutamate receptor belongs to the G-protein or second messenger-linked class of receptors and are known as “metabotropic” glutamate receptors (mGluRs). The metabotropic receptors are coupled to multiple second messenger systems that lead to enhanced phosphoinositide hydrolysis, activation of phospholipase D, increases or decreases in cyclic adenosine monophosphate (cAMP) formation, and changes in ion channel function (Schoepp D. et al, 1993; Trends in Pharmacological Science 14:13). Both types of receptors appear not only to mediate normal synaptic transmission along excitatory pathways but also to participate in the modification of synaptic connections during development and throughout life. Research has shown that mGluRs are implicated in a number of normal as well as pathological mechanisms in both the central nervous system and the periphery. Activation of neuronal mGluRs can influence levels of alertness, attention and cognition, protect nerve cells from excitotoxic damage resulting from ischemia, hypoglycemia and anoxia, modulate the level of neuronal excitation, influence central mechanisms involved in controlling movement, reduce sensitivity to pain, and reduce levels of anxiety.
Eight different types of the mGluRs have been identified: mGluR1-8 (Knopfel et al., 1995, J. Med. Chem., 38, 1417-1426). These receptors function to modulate the presynaptic release of glutamate, and the postsynaptic sensitivity of neuronal cells to glutamate excitation. Based on pharmacology, sequence homology, and the signal transduction pathway that they activate, the mGluRs have been sub-classified into three groups. Group I consists of mGluR1 and mGluR5. They are coupled to hydrolysis of phosphatidylinositol (PI) and are selectively activated by (R,S)-3,5-dihydroxyphenylglycine (Brabet et al. 1995; Neuropharmacology, 34, 895-903). Group II consists of mGluR2 and mGluR3 receptors. They are negatively coupled to adenylate cyclase and are selectively activated by (2S,1′R,2′R,3′R)-2-(2,3-dicarboxycyclopropyl)glycine (DCG-IV; Hayashi et al. 1993; Nature, 366, 687-690). Group III consists of mGluR4, mGluR6, mGluR7 and mGluR8. They are also negatively coupled to adenylate cyclase and are selectively activated by (L)-2-amino-4-phosphonobutyric acid (L-AP4).
Antagonists which selectively bind to the mGluRs have been reported. Some phenylglycine derivatives, for example S-4CPG (S-4-carboxyphenylglycine), S-4C3HPG (S-4-carboxy-3-hydroxyphenylglycine) and S-MCPG (S-.alpha.-methyl-4-carboxyphenylglycine) have been reported to antagonize trans-ACPD-stimulated phosphoinositide hydrolysis and thus possibly act as antagonists at mGluR1 and mGluR5 subtypes (Thomsen, C. et al 1993; Eur. J. Pharmacol. 245:299). More recently, compounds exhibiting selective agonist or antagonist activity at the mGluRs have been reported. Group I receptors (mGluR1 and mGluR5) play a key role in the central sensitization of pain, in addition to a variety of functions with potential implications in neurological and psychiatric disorders. (Schoepp, D. et al 1999; Neuropharmacology 38:1431-1476; Han, J. et al 2005; Pain 113:211-222.) A number of behavioral and electrophysiological studies have demonstrated a specific role for Group I mGluRs, and in particular mGluR1 receptors, in nociceptive processing in the CNS, including mechanisms of hyperalgesia and inflammation (Bhave, G. et al 2001; Nat. Neurosci. 4:417-423; Dolan, S. et al 2002; Neuropharmacology 43:319-326; Dolan, S. et al 2003; Pain 106:501-512; Young, M. et al 1994; Neuropharmacology, 33:141-144; Young, M. et al 1997; Brain Res. 777:161-169). The mGluR1-active compounds are also implicated in the treatment of pain. Antagonists at the Group 1 mGluRs antagonize sensory synaptic response to noxious stimuli of thalamic neurons (Eaton, S. A. et al. 1993; Eur. J. Neuroscience, 5:186). The intrinsic activation of spinal mGluR1 in chronic nociception has been demonstrated using antagonists, antibodies, and antisense oligonucleotides. Intrathecal administration of an mGluR1 antagonist produced antinociceptive effects in a formalin-induced model of nociceptive behavior (Neugebauer, V. 2001; Trends Neurosci. 24:550-552). There is mounting evidence to suggest that mGluR1 antagonists can be used for the treatment of chronic pain (Neugebauer, V. et al 2002; Expert Opin. Ther. Targets 6:349-361; Swanson, C. et al 2005; Nat. Rev. Drug Discovery 4:131-144). Several groups have reported a variety of structurally diverse non-competitive allosteric mGluR1 antagonists, such as LY456066 (Ambler, S. et at WO2001032632), JNJ16259685 (Mabire, D. et al 2005; J. Med. Chem., 48:2134-2153), A-841720 (Zheng, G. et al 2005; J. Med. Chem. 48:7374-7388), and R214127 (Maibre et at WO 02/28837).
The use of compounds active at the mGluRs for the treatment of epilepsy was demonstrated by the influence of trans-ACPD on the formation of convulsions (Sacaan et al, Neuroscience Lett. 139, 77, 1992) and that phosphoinositide hydrolysis mediated via mGluR1 is increased after convulsion-causing stimulation experiments in rats (Akiyama et al. Brain Res. 569, 71, 1992). Trans-ACPD has been shown to increase the release of dopamine in the rat brain, which indicates that compounds acting on the mGluRs might be usable for the treatment of Parkinson's disease and Huntington's Chorea disease (Sacaan et al., J. Neurochemistry 59, 245, 1992).
Trans-ACPD has also been shown to be a neuroprotective agent in a medial cerebral artery occlusion (MCAO) model in mice (Chiamulera et al. 1992; Eur. J. Pharmacol. 216:335), and it has been shown to inhibit NMDA-induced neurotoxicity in nerve cell cultures (Koh, V. 1991, Proc. Natl. Acad. Sci. USA 88:9431).
Compounds active at the mGluRs for treatment of neurological diseases such as senile dementia have also been reported (Zheng, G. et al 1992; Neuron 9:163; Bashir et al 1993; Nature 363:347). These studies demonstrated that activation of mGluRs is necessary for the induction of long-term potentiation (LTP) in nerve cells of the septal nucleus and hippocampus. In addition it was shown that long-term depression (LTD) in nerve cells is induced after activation of mGluRs in cerebellar granule cells (Linden et al. 1991; Neuron 7:81). mGluRs may also be involved in addictive behavior, alcoholism, drug addiction, sensitization and drug withdrawal (Wickelgren, I. 1998; Science, 280:2045).
In addition to involvement in disorders of the central and peripheral nervous system, mGluRs have recently been implicated as contributing to the development of certain cancers. In recent years, glutamate signaling in cancer has been a focus of investigation. Studies have implicated the involvement of glutamate signaling in tumor development through mGluRs. The role of glutamate signaling in non-neuronal tissues is poorly understood, but studies have shown that a variety of G protein-coupled receptors and G proteins, including those that signal through phosphoinositide hydrolysis and cAMP accumulation, have been implicated in tumorigenesis through either mutational activation or overexpression (Dhanasekaran, N. et al, 1995; Endocr. Rev. 16:259-270; Gutkind, J. 1998; Oncogene 17:1331-1342.). Glutamate has recently been linked to tumor growth in both neuronal and non-neuronal cancers (Takano, T. et al. 2001; Nat. Med. 7:1010-1015; Rzeski, W. et al 2001; Proc. Natl. Acad. Sci. USA 98:6372-6377). Glutamate has been shown to stimulate proliferation of lung carcinoma cells in serum-deprived media, and antagonists of the ionotropic AMPA and NMDA glutamate receptors have been shown to inhibit proliferation and increase cell death in a calcium-dependent manner in a variety of non-neuronal cancers (Rzeski, W. et al 2001; supra). Agonist stimulation of mGluR5 in subconfluent melanocyte cultures has been shown to result in melanocytic proliferation (Frati, C. et al. 2000; J. Cell. Physiol. 183:364-372). It was recently shown that transgenic mice bred to be predisposed to develop multiple melanomas expressed an abundance of mGluR1 in melanoma cells but not in normal melanocytes, and that ectopic expression of mGluR1 was sufficient to cause melanoma (Pollack, P. et al 2003; Nature Genetics 34:108-112). The same study revealed that mGluR1 expression was detected in several human melanoma tumors and cell lines but not in benign nevi (clusters of melanocytes on the skin) or melanocytes. Several cell lines have been developed from independent mouse melanoma tumors (Marin, Y. et al 2005; Neuropharmacol. 49:70-79). These cell lines are useful tools in the studies of signaling events that may be mediated by mGluR1 in transformed melanocytes. In these cells, stimulation of mGluR1 by quisqualate, a Group I competitive glutamate receptor agonist, results in inositol triphosphate (IP3) accumulation, and the activation of the extracellular signal-related protein kinase 1/extracellular signal-related protein kinase 2 (ERK1/2) cell signaling pathway. The extracellular signal-regulated kinase (ERK) signaling pathway is a major determinant in the control of diverse cellular processes such as proliferation, survival, differentiation and motility. This pathway is often up-regulated in human tumors and as such represents an attractive target for the development of anticancer drugs. Because of its multiple roles in the acquisition of a complex malignant phenotype, specific blockade of the ERK pathway is expected to result in not only an antiproliferative effect but also in antimetastatic and antiangiogenic effects in tumor cells. IP3 accumulation and ERK1/2 activation were inhibited by pretreatment of the tumor cells with a mGluR1-specific antagonist (S-2-methyl-4-carboxy-phenylglycine, LY367385) or by dominant negative mutants of mGluR1, demonstrating that stimulation of mGluR1 initiates the ERK pathway but that this action may be inhibited by an antagonist. It was shown that ERK1/2 activation by mGluR1 was PKC-dependent, but cAMP and PKA-independent. These results suggest that mGluR1 and glutamate signaling may be used as novel targets for melanoma therapy (Nankoon et al, 2007 Cancer Res. 67:2298-2305).
Several diseases and disorders are mediated by improper functioning of glutamate receptors. Because excitatory amino acid receptors in general and mGluRs in particular are implicated in diverse normal physiological processes, there is a need to identify compounds capable of modulating receptor-mediated functions. For example, partial antagonism of mGluRs might be clinically useful in treating disorders wherein the process mediated by the receptor is pathologically enhanced. Partial antagonism might be clinically useful in treating disorders wherein the is an overabundance of the indigenous ligand stimulating the receptor to induce a critical function. There is a need to identify and develop methods of treatment and methods of prevention of disorders related to the improper functioning of mGluRs and specifically those in Group I.